In continuous ink jet printers, streams of uniformly spaced ink drops are created by imposing predetermined vibrations upon liquid ink filaments issuing from an orifice plate. The filaments are formed by supplying ink under pressure to a print head cavity that is in communication with the orifice plate. Information is imparted to the droplet streams by selective non-charging or charging and deflection of droplets. A portion of the droplets pass to the recording medium but there are a substantial number of non-printing droplets that are intercepted by a catcher for recirculation. Usually the print head cavity has an ink return outlet (e.g. to facilitate dynamic pressure control within the cavity at start-up), and the printer's ink supply system also recirculates such return ink flow.
In addition to several other parameters, including stimulation frequency and ink viscosity, accuracy of the drop placements by a continuous ink jet printer is importantly dependent on maintaining a highly uniform ink-ejecting pressure for forcing ink streams through the print head orifices. Ejection pressure deviations will cause errors in drop velocities and, ultimately, placement error of printing drops. One way to control the ink ejecting pressure in a continuous ink circulation system is to detect the pressure of the ink in the return line from the print head and adjust the ink supply pump so that the desired pressure exists in a steady state condition (wherein some supplied ink is issuing from the print head orifices and some supplied ink is passing through the print head to the return line and its sensor).
When continuous ink jet printing is effected with a stationary print head (e.g. the print media movement providing the printing scan), the above-described pressure control system is adequate (from the pressure parameter viewpoint) to assure highly accurate drop placements. However, there are highly useful continuous ink jet printer configurations wherein the print head moves, e.g., wherein a print head having a line width array of nozzles is sequentially indexed with respect to a print media, rotating on a drum, to print sequential lines on that medium. In those configurations, we have found that the accelerations and decelerations incident to movement of the print head can give rise to drop placement errors. More particularly we have determined that line indexing of the print head and its umbilical ink conduits cause accelerations and decelerations of ink therein, which in turn give rise to ink pressure variation, drop velocity variations and thus drop placement errors.
U.S. Pat. Nos. 4,347,524 and 4,575,738 illustrate examples of the known technique of reducing shock pulses by providing a flow restrictor and a fluid reservoir in the supply line of drop-on-demand printers. Such systems function in the manner of a series-resistor, shunt-capacitor RC network to attenuate pressure pulses in the supply line and thus prevent air ingestion and/or premature drop ejection. As explained in the U.S. Pat. No. 4,575,738, the capacitive component of such a system is best effected by a chamber portion having a compressible gas (e.g. air) and ink interface. However, the gas/ink interface construction has heretofore suffered the difficulty that the ink absorbs the gas and decreases the system capacitance. Therefore, resilient membranes have been used, either instead of the compressible gas, or to separate the ink from the compressible gas. However, neither of these alternatives are as desirable as the unseparated gas/ink interface system.